The present invention relates to a method and apparatus for reading imaging plates, particularly the latent image stored in a photo-stimulated luminescence imaging plate. The invention is particularly useful for reading X-ray mammograph imaging plates, and is therefore described below with respect to this application.
Photo-stimulated luminescence imaging plates are widely used in general X-ray radiographic examination systems. Such imaging plates can be used to conveniently produce a digitized image, thereby enabling the image to be stored, enhanced, or otherwise processed according to digital techniques. In addition, such imaging plates are reusable and thereby eliminate the need for using high cost photographic film. Moreover, they substantially reduce the time for analysing the results of the examination. Because of these advantages, they are increasingly replacing the use of photographic film.
When using such imaging plates, penetrating X-rays pass through the patient and are absorbed on the surface of the imaging plate. The imaging plate records the intensity of the radiation as a proportional density of trapped electrons. To read the imaging plate, it is illuminated with light of a first wavelength (e.g., red light) which liberates the trapped electrons. Upon returning to their original sites they emit light of a second wavelength (e.g., blue light). An optical collection apparatus transmits only the second (blue) wavelength light to a detector, typically a sensitive photomultiplier tube.
The first wavelength light (e.g., red light of long wavelength) used in the readout process is typically a narrow beam of laser light focussed to a spot on the imaging plate. The spot is swept over the area of the plate. The photomultiplier tube output signal is sampled by an A/D (analog-to-digital) converter circuit at intervals which determine the pixel size of the digitally stored picture. In general, the laser beam is scanned rapidly along one coordinate, while the imaging plate is moved slowly along the other coordinate to cover the whole area. However, the imaging plate may be moved rapidly, and the scanning beam may be moved slowly. The imaging plate is erased by overall exposure to the longer wavelength (red) light and may be reused.
Sensitivity, contrast, and spatial resolution in photo-stimulated luminescence imaging depend upon the characteristics of both the imaging plate and the reader. In mammography, it is important to detect small X-ray absorbing features such as calcifications, and to detect larger masses having X-ray absorption that differs only slightly from background fatty tissue. Imaging plate materials that are capable of meeting the stringent requirements of mammography are available. However, the deficiencies of the present art of imaging plate readers have prevented use of imaging plates in mammography.
Imaging plate systems, and particularly the readers used in such systems, are described in a number of publications, including: Y. Amemiya, et al., Nuclear Instruments and Methods in Physics Research, A266 (1988) pp. 645-653; and H. Kato, Photostimulable Phosphor Radiography Design Considerations, in American Association of Physicists in Medicine, Medical Physics Monograph No. 20 (1991) pp. 731-770. Such systems and/or readers are also described in a number of patents, including: U.S. Pat. Nos. 3,859,527, 4,258,264, 4,721,856, 4,775,791, 5,081,357, 5,124,558, 5,124,913, 5,142,557, 5,365,562, 5,404,024. While some of the foregoing patents describe readers for reading X-ray mammograph imaging plates, insofar as we are aware none has yet found widespread use in X-ray mammography because of the very stringent requirements for this use of imaging plates.